1. Field of the Invention
The present invention relates to display devices and can be applied to a display device that employs current-driven light-emitting elements such as organic electro luminescence (EL) elements. In an embodiment of the present invention, basic reference voltages are divided by resistors to thereby produce plural reference voltages, and voltages are selected from the plural reference voltages for digital-analog conversion processing of image data. In particular, at least the basic reference voltage for the black level is shared by the respective color data, and the basic reference voltages for setting of an intermediate grayscale closer to the black level can be varied individually for each color data. This feature can prevent contrast deterioration due to floating black (unfavorably-bright black) and sinking black (unfavorably-dark black) with a simple configuration.
2. Description of the Related Art
In a related-art display device such as a liquid crystal display, a driver for driving a liquid crystal display panel is provided with a gamma correction circuit, and the signal level of an input signal is corrected by this gamma correction circuit to thereby assure a desired gamma. The gamma γ is expressed by Equation (1), in which IN indicates the signal level of an input signal and Y indicates the output luminance value. In a typical display device, the gamma γ is set to 2.2.IN∝Yγ  Equation (1)
Regarding the gamma correction in liquid crystal displays and so on, various improvements have been proposed in Japanese Patent Laid-open No. 2000-324508 and so on.
FIG. 15 is a connection diagram showing the configuration of one pixel in a display device employing organic EL elements. In the display device employing organic EL elements, pixels 1 are arranged in a matrix to thereby form a display part for displaying images.
In the pixel 1, a series circuit of a drive transistor Tr2 formed of e.g. a p-channel MOS transistor and an organic EL element 2 is provided between supply voltages VDD1 and VSS1. In the pixel 1, the gate of the drive transistor Tr2 is connected via a transistor Tr1 to a signal line sig. When the transistor Tr1 is turned on by a control signal VSCAN1, the gate of the drive transistor Tr2 is connected to the signal line sig, so that the potential of the signal line sig is held in a capacitor CS1 connected to the gate of the drive transistor Tr2. The drive transistor Tr2 drives the organic EL element 2 with the gate voltage dependent upon the potential of the signal line sig held in this capacitor CS1. Based on this driving, the pixel 1 causes the organic EL element 2 to emit light with the luminance value dependent upon a data voltage VDATA applied to the signal line sig.
The light emission characteristic of the organic EL element 2 is expressed by Equation (2), in which L indicates the light emission luminance value of the organic EL element 2 and I indicates the current value of the organic EL element. In Equation (2), β is represented by the equation β=μ·Cox·W/L, in which μ is the mobility of the drive transistor Tr2, Cox is the unit capacitance of the gate oxide film of the drive transistor Tr2, W is the gate width of the drive transistor Tr2, and L is the gate length of the drive transistor Tr2. Furthermore, Vdata indicates the data voltage (the signal level of an input signal), and Vth indicates the threshold voltage of the drive transistor Tr2.L∝I=β/2·(Vdata−Vth)2  Equation (2)
Applying Equation (2) to Equation (1) makes it apparent that the gamma γ of the organic EL element 2 is 2.0. Consequently, the display device employing the organic EL elements 2 can display images with a substantially appropriate gamma without provision of a gamma correction circuit.
However, in a practical organic EL element 2, the luminance value on the black side often deviates to a higher value from its ideal characteristic expressed by Equation (2), which leads to contrast deterioration. Hereinafter, this phenomenon will be referred to as floating black.
Specifically, in the display device employing the organic EL elements, a thin film transistor (TFT) is used as the drive transistor Tr2. The IV characteristic of the TFT in the saturation region is expressed by Equation (3). FIG. 16 is a characteristic curve diagram showing the IV characteristic of the TFT. In Equation (3), Ids indicates the drain current and Vgs indicates the gate-source voltage.Ids=β/2·(Vgs−Vth)2  Equation (3)
However, the TFT involves the subthreshold region in a lower-current region in particular. As shown by the dashed line in FIG. 16, the IV characteristic often deviates from the ideal characteristic expressed by Equation (3) in this subthreshold region. As a result, the floating black phenomenon will occur in the display device employing the organic EL elements.
The IL characteristic of the organic EL element 2 is expressed by Equation (4). In Equation (4), L indicates the luminance value, I indicates the current, and φ indicates the efficiency.L=φ·I  Equation (4)
The efficiency φ is a constant ideally. However, in practice, it often changes depending on the current, and very often changes in a lower-current region in particular. As this change, the efficiency decreases as shown in FIG. 17 in most cases. When the efficiency decreases on the lower current side, the black side sinks (becomes darker), contrary to the floating black, and thus the contrast deteriorates.